Not applicable.
Not applicable.
The present invention relates to methods and apparatus for monitoring characteristics of a flow stream in a pipeline. More precisely, the present invention relates to the novel application of using a flow conditioner in conjunction with a flowmeter to measure and compare several properties of a flow stream, including volumetric flow rate, speed of sound, and density of the flow stream.
Pipelines transport a large percentage of the liquid and gaseous fossil fuel products used in the world today. It is critical to both industry operations and fiscal accountability to accurately monitor and meter these products as they are transported through pipeline systems. Therefore, pipeline monitoring and metering operations must be accurate, reliable, and cost effective over a wide range of conditions.
One of the most common flowmeters in use today is the orifice meter. FIG. 1 depicts an orifice meter 50 comprising a fitting 52 having ends 54, 56 for installing the meter 50 directly into the piping section 70, typically by bolting or welding. Housed internally of the fitting 52 is a thin plate 58 that extends.across the diameter of the piping section 70, oriented perpendicular to the direction of the flow stream 75, as indicated by the flow arrows. The thin plate 58 includes a bore or opening (orifice 55), that is typically concentric, but may also be eccentric.
In operation, when the flow stream 75 reaches the orifice plate 58, the flow is forced through the orifice 55, thereby constricting the cross-sectional flow area. Due to the principal of continuity, the mass flow rate entering the orifice 55 must equal-the mass flow rate exiting the orifice 55. Therefore, because the cross-sectional area of flow is reduced at the orifice 55, the flow velocity through the orifice 55 increases to maintain the mass flow rate. Further, due to the principle of the conservation of energy, because the velocity of the flow increases through the orifice 55, the corresponding pressure must decrease.
Thus, the volumetric flow rate (Qxcex94p) through an orifice 55 having a small diameter (d) within a piping section having a larger diameter (D) is given by:                               Q                      Δ            ⁢                          xe2x80x83                        ⁢            p                          =                              CEA            o                    ⁢                                                    2                ⁢                                  xe2x80x83                                ⁢                Δ                ⁢                                  xe2x80x83                                ⁢                                  p                  o                                                            ρ                k                                              ⁢                      xe2x80x83                    ⁢          where                                    (        1        )                                E        =                              1                                          1                -                                  β                  4                                                              =                      velocity            ⁢                          xe2x80x83                        ⁢            of            ⁢                          xe2x80x83                        ⁢            approach            ⁢                          xe2x80x83                        ⁢            factor                                              (        2        )                                                      A            o                    =                                    π              ⁢                              xe2x80x83                            ⁢                              d                2                                      4                          ,                  xe2x80x83                ⁢        and                            (        3        )                                          β          =                      d            D                          ,                  xe2x80x83                ⁢                  and          ⁢                      xe2x80x83                    ⁢          where                                    (        4        )            
C is the discharge coefficient, which is a function of xcex2 and the Reynolds number, and C≈0.6. When an orifice meter 50 is used to measure volumetric flow rate (Qxcex94p), the differential pressure (xcex94p1o) across the orifice plate 58 is measured utilizing a differential pressure transducer 60. The transducer 60 is connected across the orifice plate 58 via upstream pressure tap 62 and downstream pressure tap 64 to measure the differential pressure (xcex94po). Further, a known value of density (xcfx81k) of the flow stream is provided, which may be determined using the pressure and temperature of the flow stream and compressibility data compiled and published by a standards-producing agency such as the American Gas Association (AGA) or the American Petroleum Institute (API). Alternatively, the density (xcfx81k) value may be measured online using a device, such as a densitometer (not shown). Then, from the above equations (1) through (4), using a known value of C, and calculated values for E and Ao, the volumetric flow rate through the orifice (Qxcex94p) can be calculated.
Another type of flowmeter commonly utilized today is the ultrasonic flowmeter. Ultrasonic flowmeters determine flow stream properties by transmitting ultrasonic waves across a known path length through the flow stream, receiving the ultrasonic waves, and measuring the transit time for those waves to travel across the known path length. The transit time of the ultrasonic waves are then used to determine the velocity of the fluid. As shown in FIG. 2, a typical ultrasonic flowmeter has at least two opposing transducers 20, 30 that are oriented at an angle (xcex1) to the direction of the flow stream 25, as indicated by the flow arrow. Ultrasonic waves are transmitted from transducer 20 toward transducer 30 along flow path 22 and from transducer 30 toward transducer 20 along flow path 32. The transit time of the ultrasonic waves in each direction is recorded by a processor (not shown). The two transit times, t1 and t2, are represented by the following equations:                               t          1                =                              L                                          c                us                            +                                                V                  us                                ⁡                                  (                                      X                    L                                    )                                                              ⁢                      xe2x80x83                    ⁢          and                                    (        5        )                                                      t            2                    =                      L                                          c                us                            -                                                V                  us                                ⁡                                  (                                      X                    L                                    )                                                                    ,                            (        6        )            
where cus is the speed of sound in the flow medium, X is the distance between the transducers parallel to the flow direction, as shown in FIG. 2, L is the straight line distance between the two transducers, as shown in FIG. 2, and Vus is the average velocity of the flow. Equation (5) for transit time t1 includes a positive velocity term due to the flow path 22 of the ultrasonic wave being generally in the same direction as the direction of flow 25. In contrast, equation (6) for transit time t2 includes a negative velocity term due to the flow path 32 of the ultrasonic wave being generally opposed to the direction of flow 25. Solving equations (5) and (6) simultaneously for the two unknowns (Vus and cus) yields:                               V          us                =                                            L              2                                      2              ⁢              X                                ⁢                      (                                                            t                  2                                -                                  t                  1                                                                              t                  1                                xc3x97                                  t                  2                                                      )                    ⁢                      xe2x80x83                    ⁢          and                                    (        7        )                                          c          us                =                              L            2                    ⁢                      (                                                            t                  1                                +                                  t                  2                                                                              t                  1                                xc3x97                                  t                  2                                                      )                                              (        8        )            
Therefore, for a single pair of ultrasonic transducers 20, 30, the average velocity of the flow stream (Vus) and the speed of sound in the flow stream (cus) can be determined by knowing the geometric configuration of the transducers relative to the piping (X, L) and measuring the transit times (t1, t2) of the ultrasonic waves. Ultrasonic meters have the advantage of providing flow stream data without obstructing the flow through the pipeline. Examples of ultrasonic flowmeters are shown and described in U.S. Pat. No. 4,646,575 and U.S. Pat. No. 5,546,812, both of which are hereby incorporated herein by reference for all purposes.
Velocity of a flow stream as it moves through a pipeline can be determined by an ultrasonic meter, as described above, or by other types of velocity meters, such as turbine, vortex, or electro-magnetic velocity meters. For any such velocity meter, once the velocity of the flow stream (Vvm) is determined, and the cross-sectional area of the pipe (Ap) is calculated, the volumetric flow rate (Qvm) can be determined from the following equation:
Qvm=Vvmxc3x97Apxe2x80x83xe2x80x83(9)
The reliability of any flowmeter depends upon the quality of the flow stream being measured. To provide the most accurate and reliable measurements, the flow stream should be fully developed with a symmetric velocity profile. The flow stream should also be free of swirls and other flow anomalies. An ideal, fully developed flow stream is only achievable in closely controlled laboratory situations, but such conditions can be approximated in industrial applications using a few known methods, either alone or in combination. All of the methods used to approximate a fully developed flow involve isolating the flowmeter from any disturbances caused by pipeline features such as bends, variations in piping diameter, or other meters.
One method used to approximate a fully developed flow stream is to provide a long, straight length (run) of pipe upstream of the flowmeter. Any pipeline-created anomalies will dissipate as the flow stream travels through this long run of pipe. However, the lengths of straight pipe required to sufficiently develop the flow can be in excess of one hundred times the diameter of the pipe. Therefore, flow conditioners have been developed to shorten the distance of straight pipe required to approximate a fully developed flow. Flow conditioners generally comprise a series of restricted flow paths and settling chambers that function to decrease or eliminate pipeline-induced disturbances as the flow moves through the flow conditioner. Some flow conditioners reduce disturbances in the flow so effectively that as few as seven pipe diameters of straight pipe are required between the end of the flow conditioner and the flowmeter. Thus, in order to provide suitable flow quality in the shortest possible piping run, metering systems commonly include flow conditioners. Examples of flow conditioners are shown and described in U.S. Pat. No. 5,529,093, U.S. Pat. No. 5,495,872, U.S. Pat. No. 5,762,107, U.S. Pat. No. 3,840,051, and U.S. Pat. No. 2,929,248 all of which are hereby incorporated herein by reference for all purposes.
It is also desirable to measure the density of the fluid flowing though a pipeline in order to monitor the characteristics of the fluid and adjust volumetric flow measurements accordingly. Density is often determined using compressibility data compiled and published by a standards-producing agency such as the American Gas Association (AGA) or the American Petroleum Institute (API). This compressibility data may be provided, for example, in a commercially available software package that determines the density of a known fluid based on pressure and temperature inputs. The most common of these computer applications is known as AGA-8 and is based on the AGA Report No. 8 entitled xe2x80x9cCompressibility of Natural Gas and Other Related Hydrocarbon Gases.xe2x80x9d
Pipeline monitoring and metering stations often include multiple flowmeters and other instrumentation to ensure accurate and reliable measurement of the flow rate through a pipeline. Typically, a primary meter is used for custody transfer purposes and one or more additional flowmeters are used as check meters to verify the measurements made by the primary meter. The check meters provide backup data, allowing pipeline operators to determine if changes in the flow rate measured by the primary meter result from actual pipeline flow rate changes or from a malfunction in the primary measuring device.
The present invention seeks to provide a less expensive alternative to present systems of multiple flowmeters, while retaining the functions of a check meter.
The methods and apparatus of the present invention include a measurement system having a flow conditioner, a primary flowmeter, and a processor. The processor uses at least two different methods to calculate the flow rate and density of the flow stream from various measurements, including the measured differential pressure across the flow conditioner. The processor compares and checks the calculated quantities against measured data. Thus, the present invention includes a combination flow conditioner, flow meter, and check meter. Therefore, the present invention finds a beneficial use in improving the accuracy and reliability of flow measurement while decreasing the capital costs of a measurement system.